The present invention relates to an emission electron source using nano-structures as emitters and self-aligned and nano-sized gate aperture for low voltage control, the fabrication method thereof and its use in flat panel display.
There has been broad interest in the field emission application of nano-structured materials because of their inherited high aspect ratio. Among various materials, Carbon nano-tube (CNT) appears the most promising. Numerous studies have reported the advantageous emission properties of CNT. It has a threshold field as low as 1 to 2×106 V/m, a low electron energy spread, and can carry a current as high as 500 nA (nano-Amperes) per tube. Its extraordinary high electrical and thermal conductivity as well as strong chemical and mechanical stability make it an ideal field emitter.
Potential applications of a CNT field emission device include flat panel displays, 2D sensors, direct writing e-beam lithography, microwave amplifiers, x-ray generator, electron microscopy, as well as various other instrumentations. However, all of these applications demand the device to meet part or all of the following requirements: 1) ability to modulate electron emission at a low voltage, ideally, less than 10V; 2) high emission current density; 3) high emission uniformity over large area; 4) high energy efficiency; 5) resistance to ion bombardment; 6) chemical and mechanical robustness; 7) generation of collimated electron beam. The importance of the ability to modulate a source at low voltage can hardly be over stressed. The cost of a device driver, which often is a major cost component, power consumption, as well as device miniaturization are all depend on the modulation voltage. To achieve low voltage emission modulation, a gate electrode has to be fabricated in the vicinity of the emitter to control the emission. To modulate emission at a voltage less than 10 V, this distance has to be close to one micrometer or less. High emission current density and high emission uniformity requires a high emission site density and high emission homogeneity between the sites. For display applications, for example, a minimum emission site density of 106/cm2 (equivalent to 25 emitters in a 50 μm×50 μm pixel) is required. High energy-efficiency requires both a low gate current and a low modulation voltage. For a field emission device of a meaningful lifetime and stable electron emission, the emitter material as well as device structure have to be chosen in order that the emitter is able to sustain ion bombardment and chemical erosion from active species in the device operating environment.
Despite the superior emission properties of a single CNT, the current state of the art of a CNT electron source does not meet most of the above requirements and, therefore, has not found any product applications yet, despite the appearance of some prototype flat panel displays. The major challenges and problems include:
a) Insufficient Growth Control in CNT Length, Diameter and Spacing Between them
CNT tends to grow in a wide range of length on the same sample even in a small proximity. As a result, each CNT will have a different aspect ratio and a different distance from the gate electrode, should an integrated gate electrode be built. Those CNTs with a higher aspect ratio and proximity closer to the gate will experience a stronger extraction field and, therefore, emit electrons first and with a higher current density, and in the end, burn out faster. CNT growth requires a catalyst. The diameter and spacing between CNTs is determined by the catalyst size and spacing. With a continuous layer of catalyst, or gas phase catalytic doping, CNT grow densely, causing electrostatic screening effect between neighboring CNTs. The result is an array of highly oriented and densely packed CNT, which will hardly emit electrons. The diameter of the CNT is also random, again causing variation in aspect ratio. In prior art, the spacing between CNTs is controlled by patterning catalyst into array of dots before the CNT growth using either e-beam lithography or a mono-layer of nano polymer beads as a mask for the catalyst deposition. Both methods are difficult to implement in production. E-beam lithography is so slow and expensive that it is ill suited for any meaningful operation. Applying a mono-layer consistently over large an area is no easy task.
b) Difficulty in Fabrication of an Integrated Gate Structure
Two obstacles make the fabrication difficult. First, CNT films is sensitive to wet processes. Upon exposure to a wet agent, CNTs either stick to the substrate or to themselves, diminishing their field emission properties. Second, the gate aperture has to be aligned with CNT.
In one of prior art, shown in FIG. 1A, the gate electrode is replaced by a metallic grid mechanically mounted atop of a randomly grown CNT film. Emission modulation using such a grid typically requires a voltage on the order of 1 kV.
In another prior art, shown in FIG. 1B, a gate structure is pre-fabricated using either conventional micro-fabrication or thick film technology. CNTs are then either grown into the gate hole without further processing or screen printed into the gate holes respectively. The problem is that a group of densely grown CNTs in a single gate hole does not emit well because of the strong electrostatic effect amongst them and the variation in their length and aspect ratio. CNTs mixed with other chemicals to facilitate screen-printing into gate hole do not perform well either. In addition, deposition of CNT into the gate hole often cause short circuit between the gate and the cathode electrode, resulting in low production yield.
In a third prior art, shown in FIG. 1C, vertically oriented and mono-dispersed CNT is grown before gate fabrication and the gate aperture is self-aligned with a diameter of at least 2 micrometers. The spacing between CNTs has to be controlled to at least 5 micrometers so that there will be only one CNT in each gate hole. To achieve such a growth, e-beam lithography was used to pattern the catalyst into an array of dots with desirable spacing and dot size, even though the fabrication of a self-aligned gate aperture does not require lithography.
c) High Modulation Voltage
The diameter of a typical gate aperture fabricated by prior art is typically a few micrometers, while the one fabricated by thick film process can have diameters on the order of tens of micrometers. Therefore, emission modulation voltage for these integrally gated source will be tens to hundreds of volts. For display application, these high modulation voltages are impractical, since conventional CMOS display drivers will not be able to deliver it.
d) Low Emission Site Density
The best performing field emission films today are those randomly grown CNT without exposing further processing. Typically, they have an emission site density on the order of 5×105/cm2. For an integrally gated structure, where CNTs are grown or printed in a pre-made gate aperture, there exist large variations among the CNTs, both within the same gate hole as well as from gate hole to gate hole. These variations both in length (thus tip-to-gate distance) and aspect ratio will cause emission to be dominated by those few emitters that experience the highest local electrical field. The diameter of the each gate aperture, which is at best a few micrometers, also puts a limit on the emission site density.
e) Inhomogeneous Emission
The variation in length, diameter and spacing between CNTs discussed above will cause some emitters emit electrons easier. In a display, these dominating sites appear as bright spots on the phosphor screen. And due to higher current, these sites also burn out faster and, therefore, have a short lifetime.
f) Relatively Low Resistance to Ion Bombardment and Erosion from Residual Oxygen
Oxygen is one of the major residual gases in most field emission devices. Carbon reacts easily with oxygen, causing emitter erosion. Carbon is also relatively easy to be sputtered by ion bombardment. Accumulated redeposition of the sputtered Carbon can then causes short circuit between electrodes.
g) Highly Divergent Electron Beam
Since CNT tips tend to point to random direction, the generated electron beam is highly divergent.
h) Low Production Yield
Both growth and screen-printing of CNT into a prefabricated gate holes can often cause short circuit between cathode and gate electrodes. The strict requirements of high-resolution photolithography also contribute to lower production yield and high cost.